The invention relates to a separation stage for separating a signal that is transmitted in the blanking intervals of a video signal, and to a distortion-compensation method provided for such a signal.
Teletext is a signal known in Europe that complies with the World Standard Text Format and that is transmitted in the blanking intervals of a video signal. Once all of the picture lines of a field have been transmitted in a video signal, they are followed by a vertical synchronization pulse, which resets the tube scanning beam. The vertical synchronization pulse is followed by a blanking interval in which no picture information is transmitted. This reset pause or blanking interval is used to transmit a Teletext signal, or other data services. Each Teletext line is introduced by a horizontal synchronization pulse for the electron beam of a tube in the television set.
The Teletext signal is recovered from the video signal in the television set by means of a separation stage, the so-called slicer. The first step of this process is data separation in which the received signal is broken down into binary data. This is followed by clock recovery with the receiver being synchronized to the received Teletext signal.
The transmitted Teletext signal may be subject to interference from, inter alia, noise, group delay distortion, or attenuation distortion.
It is accordingly an object of the invention to provide a separation stage for separating a signal that is transmitted in the blanking intervals of a video signal and a distortion-compensation method involving such a signal in which the signal is compensated for distortion which overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a distortion-compensating method for correcting a signal that is transmitted during the blanking intervals of a video signal. A signal is received that has a relatively high-frequency section and that has been transmitted during the blanking intervals of a video signal. It is determined whether interference in the form of frequency-dependent attenuation is present in the received signal and a difference is obtained by comparing minima of the relatively high-frequency section of the received signal with a black level of the video signal. The interference is at least partially compensated for, only if the difference exceeds a limit value.
In the distortion-compensation method, the received signal is used to determine whether interference of a specific type is present. At least partial compensation for this interference is carried out only if it exceeds a specific limit value so that insignificant interference is ignored. Distortion compensation is thus carried out only when interference is also actually present. Furthermore, the compensation is dependent on the type of interference so that each type of interference can be compensated for optimally.
The invention is advantageous since, in practice, the type and level of interference continuously varies in the signal in which distortion-compensation is to be carried out. Severe frequency-dependent attenuation, for example, can be compensated for by a peaking filter. However, if there is no interference in the supplied Teletext signal, a peaking filter would considerably reduce the signal level.
In accordance with an added feature of the invention, there is provided an embodiment that compensates for group delay distortion. This embodiment provides for a signal, called a sample signal, to be derived from a relatively high-frequency section of the signal, and to be used to sample a relatively low-frequency section of the signal. Interference is detected by determining whether signal flanks within the low-frequency section are in a predetermined or expected position relative to the sample pulses of the sample signal.
In this way, it is possible to detect whether signal flanks of the low-frequency section are related as expected in time to the sample pulses of the sample signal. To do this, it is, of course, necessary to know the relationship with respect to time when the signal is not subject to interference.
If it is found that the signal flanks of the low-frequency section are not related as expected in time to the sample pulses of the sample signal, and if this discrepancy is greater than a limit value that can be predetermined, compensation is carried out for the group delay distortion.
In accordance with an additional feature of the invention, there is provided an all-pass filter to compensate for the group delay distortion.
In accordance with another feature of the invention, the described compensation for correcting group delay distortion is applied to a Teletext signal. The high-frequency section in this case contains synchronization pulses of the Teletext signal and its low-frequency section is part of a frame code of the Teletext signal. Both the synchronization pulses (clock run in) and its frame code are standardized for a Teletext signal. This is advantageously suitable for the described process of determining whether or not group delay distortion is present.
In accordance with a further feature of the invention, there is provided an embodiment that, instead of using an all-pass filter, uses mutual phase-shifting to compensate for the frequency-dependent attenuation. The compensation is carried out by adapting the frequency of the sample signal in such a manner that the signal flanks of the low-frequency section are then in the predetermined position relative to the sample pulses of the adapted sample signal. This is achieved by mutual phase-shifting.
In accordance with a further added feature of the invention, there is provided an embodiment to compensate for frequency-dependent attenuation. With interference of this type, high-frequency signal elements are more severely attenuated than low-frequency signal elements during signal transmission. In order to detect frequency-dependent attenuation, a peak value of a relatively high-frequency section of the signal is thus compared with a reference value. If the difference between the peak value and the reference value exceeds a limit value, then it is evident that the high-frequency section has been more severely attenuated. If the presence of frequency-dependent attenuation is detected in this way, compensation is carried out for the interference, for example, by using a peaking filter which amplifies high-frequency sections of the signal more than low-frequency sections.
In accordance with a further additional feature of the invention, the reference value, with which the peak value of the high-frequency section is compared to, is a black level of the video signal.
In accordance with yet an added feature of the invention, there is provided an embodiment where the signal is once again a Teletext signal and the high-frequency section with which the frequency-dependent attenuation is detected contains synchronization pulses of the Teletext signal. As already mentioned, the synchronization pulses of a Teletext signal (clock run in) are a standardized signal section. This is a sequence of alternating zeros and ones, so that the synchronization pulses always form the section of the Teletext signal whose frequency is the highest. The synchronization pulses of the Teletext signal are thus particularly suitable both for detecting frequency-dependent attenuation and for generating the high-frequency sample signal to detect the group delay distortion according to the embodiment of the invention described hereinabove.
With the foregoing and other objects in view there is provided, in accordance with the invention, a separation stage for separating a signal that is transmitted during the blanking intervals of a video signal. The separation stage includes an input for receiving a signal that has a relatively high-frequency section with minima and that is transmitted during the blanking intervals of a video signal. A filter is provided that can be activated to compensate for frequency-dependent attenuation of the signal. A measurement apparatus is provided for detecting frequency-dependent attenuation of the signal. The measurement apparatus is connected to the input. The measurement apparatus has an output for only activating the filter when the measurement apparatus detects that a difference between the minima of the relatively high-frequency section of the signal and a black level of the video signal exceeds a limit value.
The measurement apparatus uses the received signal to determine whether the signal is subject to interference of a specific type.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a separation stage for separating a signal that is transmitted in the blanking intervals of a video signal, and a distortion-compensation method involving such a signal, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.